Immune responses to human cytomegalovirus (HCMV) infection in immunocompetent hosts present paradoxes with potentially devastating ramifications for those without immune competency. The long quest for a protective HCMV vaccine has been impeded by multiple factors, including two contradictions about HCMV natural history. (1) HCMV is considered to be a virus with low disease potential in immunocompetent hosts. Yet, the virus efficiently maintains a lifelong persistence in the presence of those immune responses that limit clinical outcomes. A hallmark of HCMV persistence is the reactivation of latent viral genomes and shedding of virus. Horizontal transmission represents an infectious threat to those at-risk for primary HCMV infection, particularly fetuses borne by mothers without prior HCMV immunity. Accumulating evidence highlights another ambiguity about HCMV immunity. (2) The immune responses to HCMV generated during primary and long-term infection, which protect against viral sequelae, are incompletely protective against reinfection with horizontally transmitted virions. It is well- established that women with prior immunity can be reinfected with antigenic variants of HCMV that can then be transmitted to their fetuses. This proposal hypothesizes that there is a key nexus linking virus-host interactions, persistence, and reinfection that is susceptible to vaccine- mediated intervention. Specifically, HCMV modulation of host immunity through the functionality of the HCMV-encoded interleukin-10 protein (cmvIL10), enables both viral reactivation and systemic spread of virions beyond sites of reinfection. HYPOTHESIS: post-exposure increases of neutralizing antibody (NAb) titers to cmvIL10 in HCMV-infected individuals will (1) reduce HCMV shedding and (2) increase resistance to reinfection. This study extends our work on the in vitro functionality of cmvIL10 and the in vivo modulation of host immunity by rhesus CMV (RhCMV)-encoded IL-10 (rhcmvIL10). Aim 1) Quantification of RhCMV shedding in RhCMV- infected monkeys following immunization with functionally inactive rhcmvIL10. Aim 2) Comparison of RhCMV reinfection in RhCMV-immune monkeys that differ in their NAb titers to rhcmvIL10. Aim 3) Characterization of rhcmvIL-10-induced alterations to host immunity.